We're talking about the baltic sea here. Its average depth is 55m.
Besides, it's already quite contaminated so there is a lot of environmental protection going around it.
http://upload.wikimedia.org/wikipedia/commons/e/eb/BalticSea...
As you can see, there really isn't places for the water to flow back and forth, so it's not a case of "the ocean is going to handle it"
Always consider the context.